1. Field of the Invention
The invention relates to a high-precision gear which is suitably available for a transmission for transmitting high torque for an automobile, an industrial machine or the like, for example, and a method of manufacturing a gear.
2. Description of the Related Art
In general, a gear for use in a transmission or the like has been heretofore manufactured as shown in FIG. 7. A cut material 1 is upset. Then, a blank 8 with no gear tooth, which is formed by using forging means, is cut (hobbed) in the circumferential surface thereof, whereby a form 4 with gear teeth is obtained. Then, the form 4 is gear-ground, whereby a finished product 7 is obtained. The cut gear thus formed has a flow line that is cut off by cutting, and thus the gear teeth decrease in strength. A solution such as a careful selection of the material or shot peening is consequently required.
An attempt has been therefore made to simultaneously form the gear teeth together with the blank by using the forging means, i.e., to obtain the so-called form with gear teeth. This form with gear teeth is proceeding toward practical use.
Forging means generally include hot forging and cold forging. Hot forgings are prone to change in dimension due to the wearing away of a die because the material or the die considerably changes in temperature. Cold forgings are susceptible to a variation in stress of the die or the material during pressurization. Thus, the gear teeth, which are formed simultaneously with a formation of the blank, vary widely. Moreover, the gear teeth are less precise.
Therefore, such gear teeth must be cut so as to improve their precision. More particularly, the product requiring high precision is surface-finished by additional gear grinding.
Accordingly, the gear requiring the high precision needs other processes such as the cutting and the gear grinding (polishing) in addition to forging.
Recently, a streamlining has been increasingly demanded. In the field of the manufacturing of the above-mentioned gear, a further reduction of the process is desired for the manufacturing of the gear which has been hitherto able to be manufactured by at least the forging, the cutting, the gear grinding and so on.
However, when the cutting is eliminated for this purpose, the gear grinding must compensate for the process equivalent to the eliminated cutting. Thus, the process is eliminated, but the time required to work considerably increases and consequently workability decreases. When one depth of cut is increased so as to make an attempt to reduce the working time, a problem arises, that is, the attempt causes damage to a grinding wheel.
It is an object of the invention to reduce processes and to streamline the manufacturing of a gear by increasing one depth of cut for gear grinding. A gear is formed by gear-grinding a form with gear teeth formed by forging means without cutting the form. A method of manufacturing a gear comprises: a forging step of forming a form with gear teeth, the form having a curved surface on which a corner along a tooth trace of a tooth flank is formed; and a gear grinding step of grinding the gear teeth of the form. Desirably, the gear is formed by gear-grinding a form with gear teeth without cutting the form, and the form has a curved surface on which a corner along a tooth trace of a tooth flank is formed by forging means.
Moreover, in the gear, hot forging can be used as the forging means. In the method of manufacturing the gear, the forging step can be hot forging.